Synthetic (i.e. non-natural) zeolites are of ever-increasing interest in industry, as witnessed especially by the numerous recent research studies relating to the production of ever more efficient zeolites, with increasingly simple synthetic processes that are economic and easy to perform.
In recent years, hierarchically porous zeolites (HPZ) have been the subject of numerous scientific publications and patent applications. Thus, as early as 2005, the process for synthesizing hierarchically porous zeolites with good crystallinity (pure phase, observed by XRD) was described in patent application WO 2007/043 731, using a structuring agent of organosilane type.
The product obtained after calcination comprises a zeolite network linked to a network of mesopores a few nanometers in diameter. The hydrothermal resistance of this product is much better than that of mesoporous solids of MCM-41 type, which makes it possible to envisage applications in which thermal regeneration takes place.
Other methods for preparing hierarchically porous zeolites, i.e. solids comprising a microporous network of zeolite type linked to a network of mesopores, have been developed and may be classified in the following manner (review by D. P. Serrano, Chem. Soc. Rev., (2013), 42, 4004-4035):                Post-treatment of the zeolite structures which consists in removing atoms from the zeolite network to create mesopores; this may be carried out either via acidic treatments which dealuminize the solid, followed by washing with sodium hydroxide which removes the aluminium residues formed (J. Perez-Ramirez et al., Adv. Funct. Mater., (2012), p 1-12) or via treatments that combine the action of an acid and that of a structuring agent which promotes the formation of mesopores (cf. WO 2013/106816).        “Hard templating method” or “moulding method” which consists in using a porous network (organic or inorganic) as a mould; this porous network is placed in contact with a reaction medium that can form a zeolite network via hydrothermal transformation, crystallization of the zeolite is performed and the mould is then removed either by calcination or by dissolution to generate the mesoporosity (C. J. H. Jacobsen, J. Am. Chem. Soc., (2000), 122, 7116-7117).        Zeolitization of amorphous mesoporous solids such as mesoporous silicas formed according to the sol-gel technique described by M. Matsukata et al., (Top. Catal., (1999), 9, 77-92).        Direct synthesis mentioned at the start using a structuring agent of the organosilane type, this type of structuring agent having the particular feature of having, on the one hand, affinity with the silico-alumina species which form the zeolite network by virtue of its silane function, and, on the other hand, of being able to occupy a space with its long-chain organic function which serves to occupy the space and to create mesoporosity when it is removed (patent application WO 2007/043 731).        
However, even though the solids obtained according to this direct synthetic process do indeed have hierarchical porosity as shown by the nitrogen adsorption isotherms and the transmission microscopy photos (Angew. Chem. Int. Ed., (2012), 51, 1962-1965), it is observed that:                the micropore volume of these hierarchically porous zeolites is significantly lower than that of non-mesoporous zeolites,        the structuring agent modifies the growth rates of the crystal faces, which does not allow the size of the crystals to be correctly controlled,        the increase in content of structuring agent directed towards increasing the mesopore volume leads to a loss of selectivity for the crystallization of a given zeolite, which results in the formation of an unwanted mixture of zeolite structures (Y. Meng et al., Asian Journal of Chemistry, 25 (8), (2013), 4423-4426).        
One of the objects of the present invention is to solve at least these three major drawbacks noted for direct synthesis using a structuring agent of organosilane type.
Mention may also be made of the following documents, in which the use of structuring agents of organosilane type, and of organosilane derivatives, is described, for the purpose of synthesizing various hierarchically porous zeolite structures including zeolites X and LTA.
Thus, R. Ryoo (Nature Materials, (2006), vol. 5, p. 718 sqq.) describes the synthesis of LTA having mesoporosity and, later, (K. Cho et al., Chem. Mater., 21, (2009), 5664-5673) the synthesis of mesoporous zeolites of LTA type and applications thereof in catalysis. The diffractograms presented in FIG. 2 of the article by K. Cho (see above) show that there is no contaminating crystalline phase. On the other hand, the decrease in intensity of the peaks, when there is addition of structuring agent and a fortiori when its amount increases, proves a degradation of the crystalline framework (low microporosity).
Patent application EP2592049 proposes the synthesis of a zeolite having very substantial and well-organized mesoporosity, but with a marked degradation of the crystalline framework (very low microporosity). This process uses a specific structuring agent comprising three ammonium functions.
The studies by W. Schwieger (Angew. Chem., Int. Ed., (2012), 51, 1962-1965) concern the synthesis of mesoporous zeolite of FAU (X) type using a structuring agent. A single example presents the use of TPHAC ([3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride) as structuring agent, with a TPHAC/Al2O3 mole ratio equal to 0.06.
The article by Y. Meng (Asian Journal of Chemistry, 25 (8), (2013), 4423-4426) describes syntheses of mesoporous zeolite LTA using [3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride (TPOAC), as structuring agent, and presents a study of various synthetic parameters including the amount of structuring agent used, the alkalinity of the reaction medium and the crystallization temperature.
It emerges that an increase in the content of structuring agent which should lead to an increase in the mesopore volume also has the effect of modifying the growth rates of the zeolite network, thus resulting in the appearance of other zeolite crystalline phases and thus the formation of mixtures of zeolite structures, which is not desired. Moreover, the diffractograms of FIG. 1 of the said article show a lowering of the crystallinity.
The abovementioned prior art moreover shows that the micropore volumes are markedly lower than the micropore volumes of equivalent non-mesoporous zeolites (i.e. zeolites whose mesoporous outer surface area as defined below is strictly less than 40 m2·g−1), which is very detrimental in applications in which a high content of active sites is required. What is more, the size of the crystals is subject and cannot be modified.
Finally, the preparation processes described in the prior art do not appear to be readily industrializable especially on account of the high costs that they may generate, and on account of the synthesis times, which are proportionately longer the higher the desired mesoporosity.
With regard more particularly to the mesoporous zeolites of Y type, the literature provides some references regarding the syntheses thereof involving post-treatments of zeolites Y.
Thus, for example, the article by U. Lohse et al. (Z. Anorg. Allg. Chem., 476, (1981), 126-135) describes the creation of a system of mesopores having a size close to 20 nm in a zeolite Y by steam treatment then by acid extraction. However, these successive treatments lead to a drastic reduction in the crystallinity of the mesoporous zeolite Y in comparison with the untreated initial zeolite.
In document US 2013/0183229, the post-treatment is carried out by introducing an amount of Pluronic® (nonionic surfactant) of the same order of magnitude (similar amount by weight) as the amount of zeolite Y, then by long liquid-route treatments, followed by several calcination treatments.
Documents U.S. Pat. No. 8,486,369 and US 2013/0183231 present post-treatments that use a cetyltrimethylammonium (CTA) halide coupled with an acid, then a steam treatment. However, such post-treatments have the major drawbacks of reducing both the crystallinity and the micropore volume of the initial zeolites. They also result in the material yields being drastically reduced. These effects are even more marked when the desired mesopore volume formed is large.
Another example is illustrated by the studies by D. Verboekend et al. (Advanced Functional Materials, 22(5), (2012), 916-928) which present zeolites Y having mesopores obtained by a succession of post-treatments. It is indicated (p. 919, left-hand column, 1st §) that these post-treatments greatly degrade the microporosity. This degradation of microporosity is not however visible in Table 2, due to the fact that the micropore volume is measured by D. Verboekend (ibid.) using the t-plot method that simultaneously measures the volumes of the micropores and of the small mesopores. Measurement of the micropore volume using the Dubinin-Raduskevitch equation only takes into account the micropores with a diameter that is strictly less than 2 nm (cf. “Adsorption by powders and porous solids”, F. Rouquerol et al., Academic Press, (1999), chap. 8.2.2, pages 224-225).
Application WO 2012/084276 describes a process for preparing a mesoporous zeolite Y by various basic post-treatments but to the detriment of the microporosity. These treatments furthermore result, as claimed, in an increase of the Si/Al atomic ratio via dealumination.
Although these processes make it possible to prepare hierarchically porous zeolites, as shown by the shape of the nitrogen adsorption isotherms of the solids obtained, it is important to note that these processes use amounts of post-treatment fluids of the same order of magnitude as the initial mass of zeolite with numerous long operations. Furthermore, the mass yield of these processes is less than 60%, which further penalizes their production efficiency. These processes are therefore long, expensive and relatively unproductive.
It therefore appears that these prior art documents that relate to the preparation of mesoporous Y-type zeolites only propose synthesis techniques that comprise at least one post-treatment step.
The studies by Baoyu Liu et al. (RSC Advances, 3, (2013), 15075-15084) teach the synthesis of hierarchically porous zeolites Y prepared with the aid of a sacrifical template. However, these zeolites have micropore volumes that are insufficient with regard to the targeted applications that use the said hierarchically porous zeolites Y.
Thus, one aim of the present invention consists in providing hierarchically porous Y-type FAU zeolites combining mesoporosity, high micropore volume, optimal purity and adjustable crystal sizes and with an Si/Al atomic ration which is strictly greater than 1.4. Another aim of the present invention consists in providing a process that is economical, simple and readily industrializable for the preparation of the said zeolites.